Igniter plugs for gas turbine engines are akin to spark plugs used for ignition in internal combustion engines in that both basically comprise a metal tubular outer shell, a metal central electrode extending axially through the shell and a ceramic core insulator extending coaxially with the central electrode and supporting the electrode within the shell. The differential between the thermal coefficients of expansion of the metal shell and the ceramic core on long igniter plugs and the severe conditions under which an igniter must operate have led to continuing improvements in the basic igniter plug structure for improved reliability and service life.
The problem of differential expansion between the metal shell and ceramic core was solved, in part, by constructing the core in two cylindrical pieces, one of which telescoped partly within the other. The core piece nearest the combustion chamber end of the shell was sealed to the shell near that end and the core piece extending into the opposite end of the shell was sealed to the shell near that end.
Arc erosion of the insulator at the discharge end of the igniter and subsequent loosening of the insulator frequently caused such igniters to fail prematurely from wear of the insulator due to vibration and loss of contact with the shell.
This problem was addressed in U.S. Pat. No. 3,882,338, issued May 6, 1975 to H. P. Meyer for "Igniter Plug". This patent discloses an igniter in which the forward insulator portion is continuously biased into contact with the forward end of the shell by one or more compression springs. The insulator is thereby constrained against unrestricted movement and the vibrational energy is absorbed by flexure of the spring.
The insulator material used in certain igniters is beryllia. Beryllia possesses good thermal conductivity and other properties which make its use highly advantageous so far as operational characteristics are concerned. However, beryllia is also a highly toxic material, requiring extraordinary precautions in fabrication and disposal of the product. An alternative insulator material which has many of the attractive characteristics of beryllia is boron nitride. Boron nitride is not considered a hazardous material, hence its use eliminates the primary disadvantage of beryllia.
Experimental igniters fabricated with boron nitride insulators operated satisfactorily except that igniters removed from engines before the end of their expected service life have shown excessive wear in the insulator in the area surrounding the electrode tip. Such wear is undesirable because of the increased possibility of cracking of the insulator and increased firing voltage resulting therefrom.
It has been demonstrated that the wear occurring in igniters equipped with boron nitride insulators is not due to spark erosion. Instead, unrestrained vibration of the electrode tip mechanically abrades the insulator, increasing the clearance between the electrode tip and insulator.
U.S. Pat. No. 4,309,738, issued Jan. 5, 1982 to G. F. Mulkins et al. for "Igniter Plug" discloses means for restraining vibration of the electrode tip in an igniter constructed generally as discussed above. The restraining means comprise either a cup-shaped or U-shaped sleeve recessed within the forward end of the insulator surrounding the center conductor of the igniter near the electrode tip. These means restrain vibration of the electrode tip mainly by increasing the rigidity of support. They are disadvantageous in that they require a reduction in the thickness of the surrounding insulator wall and in that they transmit the vibrational forces to the weakest portion of the surrounding insulator.
It is an object of the present invention to provide a means for reducing vibration of the center electrode in an igniter plug.
It is another object of the invention to provide, in an igniter plug, means for reducing vibration of the center electrode which are absorbent of vibrational energy without imposing appreciable load on the igniter insulator.
A further object of the invention is to provide, in an igniter plug, means for reducing vibration of the center electrode which include the ability to partially compensate for differences between the thermal coefficients by expansion of the igniter materials and for misalignments and tolerance variances in the igniter structure.
Other objects and advantages of the invention will become evident as an understanding thereof is gained from the following complete description.